1. Field of the Invention
Embodiments of the present invention relate to systems and methods for measuring the polarization state of a light field as a function of spatial position. More particularly, embodiments of the present invention relate to systems and methods for measuring the polarization state of a light field by dispersing a polarization data cube across a coding mask of a spectrometer.
2. Background Information
An imaging photopolarimeter measures the polarization state of a light field as a function of spatial position. The polarization state is most often described in terms of the Stokes parameters, but may be described on an equivalent basis, such as a coherence vector.
Imaging polarimeters have previously been constructed for diverse spectral ranges and applications. In the oldest and most conventional approach to imaging polarimetry the entire input aperture of an imaging system is covered with a temporally varying polarization element or combination of elements. This approach is called temporally variable polarization filtering. Under this approach, one might, for example, record a clear image, images through a linear polarization analyzer at various orientations, and an image through a circularly sensitive analyzer system. The relatively slow image capture time associated with temporal scanning is the primary drawback of this approach, although the cost of adaptive full aperture polarization elements is also a factor. A recent version of this approach combines polarization sensitive holographic devices with temporally modulated polarization elements.
One alternative to varying the polarization properties of a single aperture is using an array of polarization sensitive imagers. This alternative is called spatially parallel polarization imaging. According to this alternative, for example, each aperture is tuned to measure a projection on the polarization state. The full Stokes vector image is then reconstructed jointly from the array of images. This approach overcomes the challenges of the temporally modulated system. A full polarization image may be captured in a single camera frame. The disadvantage of this approach is that it is difficult to spatially register images captured from different cameras. This disadvantage can partially be overcome by using an integrated prism set to separate polarization channels, much in the same way as prism arrays are used in multiple focal plane color cameras. However, the requisite prism assembly is difficult and expensive to manufacture into polarization components, and the overall system cost for multiple camera systems is high.
Various groups have manufactured arrays of micropolarizer components, or micropolarizer arrays. These systems are similar in philosophy to color cameras with red green blue (RGB) filter arrays integrated on the focal plane. Each focal plane pixel detects a specific polarization component, and the well-registered image is synthesized by interpolation between pixel values. Micropolarizer arrays have been particularly successful in infrared imaging systems, where wire-grid fabrication technologies may be applied. Various studies have also demonstrated micropolarizer arrays and nanoscale optical components for visible systems, but to date system performance, fabrication cost, and simplicity have not been sufficiently satisfactory to justify system construction.
Imaging polarimetry has been demonstrated by interferometric sampling in polarization imaging systems. Such systems employ interferometric imaging polarimetry, for example. These systems work in visible spectral ranges, but function only for monochromatic laser illumination rather than broad spectral signals.
In view of the foregoing, it can be appreciated that a substantial need exists for systems and methods that can measure the polarization state of a light field as a function of spatial position in a single time step, at a low cost, and for broad spectral signals.